1. Field of the Invention
The present invention relates to a toner needed for an electrophotographic technique and a method for manufacturing the toner.
More particularly, the present invention relates to a toner comprising: a component consisting of an amorphous plant-derived resin having a carboxyl group; a component consisting of a crystalline epoxy resin having a glycidyl group; and a component consisting of a cross-linked resin obtained by cross-linking the carboxyl group of the plant-derived resin and the glycidyl group of the epoxy resin, and to a method for manufacturing the toner.
2. Description of the Related Art
Generally, image forming apparatuses using an image formation process of an electrophotographic system form a desired image on a medium by carrying out a series of steps including: a charge step for uniformly charging a photosensitive layer on a surface of a photosensitive drum working as a latent image carrier; an exposure step for projecting signal light of an image on a document onto the surface of the photosensitive drum in a charged state to form an electrostatic latent image; a development step for developing the electrostatic latent image on the surface of the photosensitive drum by supplying an electrophotographic toner to the electrostatic latent image; a transfer step for transferring the toner image on the surface of the photosensitive drum onto a medium such as paper and an OHP sheet; a fixing step for fixing the toner image on the medium by heating and pressurization; and a cleaning step for cleaning the surface of the photosensitive drum by removing toner and the like left on the surface of the photosensitive drum after the transfer of the toner image with a cleaning blade.
In some cases, the transfer of the toner image to the medium is performed via an intermediate transfer medium. Developers used for such image forming apparatuses include a one-component developer consisting only of a toner as the main component and a two-component developer containing a mixture of a toner and a carrier for use.
The toners used for these developers are produced by, for example, a kneading and pulverizing method or a polymerization method represented by a suspension polymerization method and an emulsion polymerization aggregation method.
In the kneading and pulverizing method, toner materials containing a binder resin and a colorant as main components, and optionally containing a release agent, a charge controlling agent, and the like added and mixed therein are melted and kneaded, cooled and solidified, and then pulverized and classified to manufacture a toner.
From the viewpoint of global environment conservation, various efforts have been made in various technical areas in recent years. Since materials of a number of manufactured articles are currently produced from petroleum, efforts to reduce carbon dioxide generated and energies needed when such materials are produced or burnt are very important from the viewpoint of prevention of global warming.
In addition, while energy saving has been also considered from various angles as another effort leading to prevention of global warming, there is a growing awareness that reduction of fixing energy by lowering fixing temperature for a toner transferred on a medium such as paper and an OHP sheet is effective in the field of electrophotography.
At the same time, copying machines and facsimile machines are desired to be further high-speed. In order to deal with such trends, lowering of the melting temperature of toner is essential.
As a method for fixing a toner image transferred onto a medium such as paper and an OHP sheet, a contact heating type fixing method is often used in which the toner image is heated, melted, and pressurized by using a heat roll or the like to fix the image on the medium.
Toner can be evaluated for the fixing ability in the contact heating type fixing method by determining a temperature width allowing fixation between a fixing temperature of the lower limit and a temperature for hot offset initiation.
The above-mentioned lowering of the melting temperature of toner means lowering of the fixing temperature of the lower limit, whereby fixation at low temperature can be achieved.
As the binder resin for toner, a resin having a cross-linked structure, a resin containing a high molecular weight substance and a low molecular weight substance, and the like are used. In this regard, on one hand, when the content of a cross-link component or a high molecular weight substance component is increased to improve the hot offset resistance in such binder resins, the melt viscosity of the resin will be too large, which may cause insufficient low-temperature fixing ability of the toner.
On the other hand, when the content of a low molecular weight substance component is increased to improve the low-temperature fixing ability, the melt viscosity of the resin will be smaller, but the elasticity of the toner will be reduced, which may cause deterioration in hot offset resistance.
Accordingly, design of the binder resin for toner is particularly important to achieve the lowering of the melting temperature of toner and maintain the offset resistance at high temperature.
In addition, use of plant-derived resources called biomass attracts a lot of attention as a new effort leading to prevention of global warming.
This is because carbon dioxide generated when the biomass is burnt is originally carbon dioxide in the atmospheric air taken by plants through photosynthesis, and it is therefore considered that the balance of the carbon dioxide in the atmospheric air is zero in total, that is, the total amount of the carbon dioxide dose not change.
The property of thus not apparently affecting increase or decrease of carbon dioxide in the atmospheric air is referred to as carbon-neutral, and it is considered that use of carbon-neutral plant-derived resources can fix the content of carbon dioxide in the atmospheric air.
Plastics produced from such biomass are referred to as biomass polymers, biomass plastics, nonpetroleum-based polymeric materials or the like, and monomers that can be materials for these plastics are referred to as biomass monomers.
In the field of electrophotography, likewise, there has been made an effort of using biodegradable resins containing biomass as resources excellent in environmental safety and effective for carbon dioxide emission reduction in consideration for global environment conservation.
To take a polyester resin as an example, which is generally produced by condensation polymerization of a dicarboxylic acid component and a diol component, there have been proposed a technique in which a polyester resin produced by using a biomass monomer such as succinic acid and itaconic acid as the dicarboxylic acid component and using a biomass monomer such as 1,3-propanediol as the diol component is used as a binder resin for color toner; and a technique in which a polylactic resin, which is a biomass polymer produced from lactic acid as a material obtained from corn or other plants is used as a binder resin for toner.
However, these plant-derived resins are aliphatic hydrocarbon compounds and therefore, when used as a binder resin for toner, cause the toner to have poor hot offset resistance.
To deal with this problem, Japanese Unexamined Patent Publication No. HEI 9 (1997)-281746 proposes a method of improving the hot offset resistance of toner by cross-linking polylactic acid, which is a plant-derived resin by an isocyanato.
In the toner produced by the method disclosed in Japanese Unexamined Patent Publication No. HEI 9 (1997)-281746, however, the low-temperature fixing ability is reduced, while the hot offset resistance can be improved.